Method for monitoring a flood front

ABSTRACT

A method of monitoring a flood front. The method may comprise installing at least one optical electromagnetic sensor in a wellbore which penetrates an earth formation. The sensor may be a part of an interferometer selected from the group consisting of a Mach Zehnder interferometer and a Michelson interferometer. The method may further comprise inducing an electromagnetic field in the earth formation. A first optical path length in a first optical waveguide of the sensor may increases in response to exposure to the electromagnetic field, and a second optical path length in a second optical waveguide of the sensor may decreases in response to exposure to the electromagnetic field. Additionally, the method may comprise monitoring the flood front by detecting via the sensor the electromagnetic field in the earth formation as the flood front progresses through the earth formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/679,940 filed Nov. 16, 2012, which is herein incorporated byreference in its entirety.

BACKGROUND

This disclosure relates generally to equipment utilized and operationsperformed in conjunction with a subterranean well and, in an exampledescribed below, more particularly provides optical push-pullinterferometric sensors for electromagnetic sensing.

It can be useful to monitor a subterranean reservoir over time, in orderto detect changes in the reservoir. For example, in conventional andenhanced oil recovery, processes such as water flooding, steam floodingand chemical flooding can be implemented. It is useful to monitorinjection of water, steam or chemicals into a formation, and/or tomonitor progress of the water, steam or chemicals toward or away fromone or more wellbores. Monitoring a flood front helps avoid floodbreakthroughs, and thereby optimize hydrocarbon production, and can alsosave costs by reducing an amount of steam, water and/or chemicals used.

Therefore, it will be appreciated that improvements are continuallyneeded in the art of monitoring changes in subterranean reservoirs. Suchimprovements may be used for monitoring flood front progress, formonitoring other changes in an earth formation, or for other purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative partially cross-sectional view of a wellsystem and associated method which can embody principles of thisdisclosure.

FIGS. 2-9 are representative views of optical electromagnetic sensorswhich may be used in the system and method of FIG. 1.

FIGS. 10-12 are representative views of multiplexing methods which maybe used with the sensors.

DETAILED DESCRIPTION

Representatively illustrated in FIG. 1 is a system 10 for use with asubterranean well, and an associated method, which system and method canembody principles of this disclosure. However, it should be clearlyunderstood that the system 10 and method are merely one example of anapplication of the principles of this disclosure in practice, and a widevariety of other examples are possible. Therefore, the scope of thisdisclosure is not limited at all to the details of the system 10 andmethod described herein and/or depicted in the drawings.

In the FIG. 1 example, the system 10 is used to monitor a flood front 12as it progresses through an earth formation 14. For this purpose,optical electromagnetic sensors 20 are installed in a wellbore 16 alongan optical cable 18. The cable 18 and sensors 20 are positioned incement 22 surrounding casing 24.

The flood front's 12 progress may be monitored toward or away from thewellbore 16, or another characteristic of the formation 14 could bemonitored, etc. Thus, the scope of this disclosure is not limited to thedetails of the example depicted in FIG. 1.

In the FIG. 1 system 10, monitoring of the flood front 12 isaccomplished by detecting changes in the formation 14 over time. This ispreferably accomplished by measuring resistivity contrasts. Theresistivity of the formation 14 is obtained through the measurement ofelectromagnetic fields in the formation using the opticalelectromagnetic sensors 20, which are preferably permanently installedin the wellbore 16, so that continuous monitoring over time isavailable.

A transmitter 26 can be used to generate electromagnetic energyconsisting of electric and magnetic field components. Thus,electromagnetic fields 28 (e.g., primary, secondary, etc., fields) areinduced in the formation 14. However, it should be clearly understoodthat the scope of this disclosure is not limited at all to anyparticular way of inducing electromagnetic fields in a formation, or toany particular type of electromagnetic fields induced in a formation.

The transmitter 26 could comprise coils external to the casing 24. Inother examples, the casing 24 itself could be used to generate theelectromagnetic fields 28, such as, by using the casing as a conductor.

In further examples, the transmitter 26 could be positioned in anotherwellbore, at the earth's surface, or in another location. The scope ofthis disclosure is not limited to any particular position of atransmitter, to any particular type of transmitter, or to any particulartechnique for generating an electromagnetic field in the formation 14.

The sensors 20 detect the electromagnetic field 28. Measurements of theelectromagnetic field 28 are then inverted to obtain the resistivity ofthe formation 14.

In some examples, a time-lapse measurement may be performed, in whichelectric or magnetic fields at each sensor 20 location are measured as afunction of time. In a time-lapse measurement system, first a sensorsignal is recorded at a time when there is no flood. During reservoirmonitoring for waterfront, a differential signal (between the no floodcase and with flood case) at each sensor is recorded—which is the fielddue to flood. As the flood approaches closer to a sensor 20 (e.g., in aproduction well), the differential signal gets larger. The intensity ofthe signal indicates a distance to the flood front 12.

The final output of the system could either be resistivity or field dueto flood, depending on a post-processing algorithm used. Direct fieldmeasurement is comparatively straightforward, while converting thedirect measurement to resistivity makes the post-processing morecomplicated.

Basically, in the sensors 20, a physical perturbation interacts with anoptical waveguide to directly modulate light traveling through thewaveguide. This modulated signal travels back along the same or anotherwaveguide to a signal interrogation system, where the signal isdemodulated, and the corresponding perturbation is determined.

Preferably, an optical fiber (or another optical waveguide, such as anoptical ribbon, etc.) is bonded to or jacketed by a ferromagneticmaterial which is a magnetostrictive material used as a magnetic fieldreceiver. Such materials undergo a change in shape or dimension (e.g.,elongation or contraction) in the presence of a magnetic field.

This property is known as magnetostriction. Some widely usedmagnetostrictive materials are Co, Fe, Ni, and iron-based alloysMETGLAS(™) and TERFENOL-D(™).

The sensors 20 can be used to measure electric fields when the opticalwaveguide is bonded to or jacketed by a ferroelectric material which isan electrostrictive material. Ferroelectric materials undergo a changein shape or dimension in the presence of an electric field.

This property is known as electrostriction. Some examples ofelectrostrictive ceramics are lead magnesium niobate (PMN), leadmagnesium niobate-lead titanate (PMN-PT) and lead lanthanum zirconatetitanate (PLZT).

However, it should be clearly understood that the scope of thisdisclosure is not limited to use of any particular magnetostrictive orelectrostrictive material. Any suitable material which changes shape inresponse to exposure to a magnetic and/or electric field may be used.

Referring additionally now to FIGS. 2-5, several examples of the sensor20 are representatively illustrated. In each of these examples, anoptical waveguide 30 is bonded or otherwise attached to a material 32which changes shape in response to exposure to an electric and/ormagnetic field.

In the FIG. 2 example, the material 32 is in the form of a wire or rodwhich is bonded to a section of the waveguide 30. For example, an epoxymay be used to adhere the optical waveguide 30 to the material 32.

When the material 32 changes shape, the length of the optical waveguide30 bonded to or jacketed by the material is elongated or contracted.Thus, strain is induced in the waveguide 30 due to the electromagneticfield 28.

In FIG. 3, the waveguide 30 is jacketed or coated (surrounded) by thematerial 32. The material 32 is bonded or otherwise adhered to an outersurface of the waveguide 30.

In FIG. 4, the material 32 is planar in form. Again, the material 32 isbonded to the waveguide 30.

In FIG. 5, the waveguide 30 is wrapped about the material 32, which isin cylindrical form. The waveguide 30 is not necessarily bonded to thematerial 32, since a radial enlargement or contraction of thecylindrical material will change strain in the waveguide 30 without suchbonding. However, the waveguide 30 could be bonded to the material 32 inthis example, if desired.

In the FIGS. 2-5 examples, a strain (or change in length per unitlength) can be induced in the waveguide 30 due to a change in shape ofthe material 32.

The strain (for magnetostrictive material 32) is given by:ε₃=CH²  (1)where C is an effective magnetostrictive coefficient, and H is a sum ofalternating and direct magnetic fields (H_(ac) and H_(dc)). Expandingthe H field term, and extracting only the term that has the samefrequency as the original H_(ac) gives:ε₃=2CH_(ac)H_(dc)  (2)This indicates that the strain is linearly proportional to the magneticfield.

Similarly, the strain (for an electrostrictive material 32) is given by:ε₃=ME²  (3)where M is an effective electrostrictive coefficient, and E is a sum ofalternating and direct electric fields (E_(ac) and E_(dc)). Expandingthe E field term, and extracting only the term that has the samefrequency as the original E_(ac) gives:ε₃=2ME_(ac)E_(dc)  (4)This indicates that the strain is linearly proportional to the electricfield.

The strain (due to magnetostriction or electrostriction of the material32) can be measured using interferometric methods, such as Mach-Zehnder,Michelson, Sagnac, Fabry-Perot, etc.

Representatively illustrated in FIG. 6 is a Mach-Zehnder interferometer36 for measuring strain. The interferometer 36 comprises a sensing arm37 and a reference arm 38. The material 32 is bonded to, jacketed about,or wrapped about the waveguide 30 which comprises the sensing arm 37 ofthe interferometer 36. The sensing arm and the reference arm 38 areconnected in parallel between two optical couplers 40.

A light source 42 (such as a laser, etc.) transmits light 44 through thesensing arm 37 and the reference arm 38. An optical detector 46 receivesthe light, which is the interference between the lights 44 from the twoarms 37, 38.

If the optical waveguide 30 undergoes strain due to exposure of thematerial 32 to an electric and/or magnetic field, this changes anoptical path length for the light 44 in the sensing arm 37 as comparedto the reference arm 38. This change in path length causes an opticalphase shift between the light 44 transmitted through the sensing arm 37and light transmitted through the reference arm 38. The phase change isgiven by:Δφ=2πnL/λ*[ε₃=[(P₁₁+P₁₂)ε₁+P₁₂ε₃]n²/2]  (5)where Δφ is a difference in phase, n is the refractive index, L is alength of the waveguide 30 bonded to, jacketed by, or wrapped about thematerial 32, λ is the wavelength of light 44, P11 and P12 are Pockelscoefficients, and ε₁ and ε₃ are strains in transverse and longitudinaldirections, respectively.

The phase difference Δφ measured using this method is proportional tothe magnetic and/or electric field, which in turn is a measure ofresistivity. Of course, other methods may be used for detecting thechange in length of the waveguide 30, and for relating this lengthchange to the electromagnetic field strength, in keeping with the scopeof this disclosure.

In the FIG. 1 example, the sensors 20 are used for monitoring the floodfront 12. The sensors 20 are deployed in the cement 22 external to thecasing 24. The sensors 20 comprise a series of equal lengths ofwaveguide 30 bonded to, jacketed by, or wrapped about the material 32 atequal spacings along the cable 18.

At the sensor 20 locations, an optical signal (such as light 44)transmitted through the waveguide 30 is modulated by a change in shapeof the material 32 due to the electromagnetic field 28. The modulatedsignal from each sensor 20 travels along the cable 18 to a signalinterrogation device, where each sensor's signal is extracted anddemodulated, enabling a determination of the electromagnetic fieldstrength at each sensor location. In this manner, resistivity of theformation 14 can be mapped along the optical cable 18.

Referring additionally now to FIG. 7, another example of the sensor 20is representatively illustrated. In this example, the waveguide 30 isbonded to the material 32 repeatedly, so that a change in shape of thematerial will result in larger strain in the waveguide.

A greater length of the waveguide 30 bonded to, jacketed by, or wrappedabout the material 32 results in a greater total strain induced in thewaveguide. This technique enhances a sensitivity of the sensor 20 to theelectromagnetic field 28.

Another technique for enhancing a sensitivity of the sensor 20 isrepresentatively illustrated in FIG. 8. This technique may be used with,for example, Mach Zehnder and Michelson interferometers, which typicallyhave a sensing arm and a reference arm.

In the FIG. 8 example, the waveguide 30 is bonded to, jacketed by, orwrapped about the material 32. Preferably, the material 32 in thisexample comprises a magnetostrictive material which contracts inresponse to exposure to a magnetic field (negative magnetostriction).Materials which experience negative magnetostriction include Ni, Co,ferrites, nickel ferrites and Co doped nickel ferrites.

Another arm of the interferometer 36 comprises another optical waveguide50 which is bonded to, jacketed by, or wrapped about another material52. Preferably, the material 52 in this example comprises amagnetostrictive material which elongates in response to exposure to amagnetic field (positive magnetostriction). Materials which experiencepositive magnetostriction include METGLAS™, Fe, PERMALLOY™ andTERFENOL-D™.

Thus, when the sensor 20 of FIG. 8 is exposed to a magnetic field, thearm comprising the waveguide 30 will contract, and the arm comprisingthe waveguide 50 will elongate. This gives a larger total difference inoptical path length between the arms, as compared to the example of FIG.6.

Since the optical path length change is greater in the FIG. 8 example, alarger phase difference will be achieved between the light 44transmitted through the waveguide 30 and the light transmitted throughthe waveguide 50. This larger phase difference can be more easilymeasured for a given magnetic field, and small changes in the magneticfield can be more easily detected, resulting in a sensing system withhigher sensitivity.

Referring additionally to FIG. 9, another example of the sensor 20 isrepresentatively illustrated. In this example, the interferometer 36comprises a Michelson interferometer, with Faraday mirrors 54 at ends ofthe waveguides 30, 50 opposite an optical coupler 40.

As with the Mach Zehnder interferometer 36 in the FIG. 8 example, theFIG. 9 Michelson interferometer has an increased difference in opticalpath length when the sensor 20 is exposed to a magnetic field, becausethe waveguide 30 contracts while the waveguide 50 elongates in responseto exposure to the magnetic field. A phase change measured by thedetector 46 will be larger for a given magnetic field, due to theincreased difference in optical path length.

Referring additionally now to FIGS. 10-12, examples of techniques formultiplexing the sensor 20 are representatively illustrated. Wavelengthdivision multiplexing (WDM), time division multiplexing (TDM) and hybridexamples are illustrated in the drawings, but it should be understoodthat any multiplexing technique may be used, in keeping with the scopeof this disclosure.

For the WDM examples, a light source is preferably a broadband laser,whereas for the TDM examples the light source is preferably pulsed. Forhybrid examples, preferably a broadband pulsed source is used.

In FIG. 10, WDM drops or filters 56 are used to transmit certainwavelengths (λ1, λ2, λ3, . . . ) to respective ones of the sensors 20.In this example, the sensors 20 include the Mach Zehnder interferometer36, but other types of interferometer (such as Michelson) may be used inother examples.

WDM adds or couplers 58 can be used to transmit all of the wavelengthsof light 44 to the detector 46 for de-multiplexing and demodulating. Thelight which passed through each sensor 20 is readily identified by itscorresponding wavelength (λ1, λ2, λ3, . . . ). Thus, FIG. 10 is anexample of wavelength division multiplexing of the sensors 20.

In FIG. 11, optical delay coils 60 are used to achieve time divisionmultiplexing. The light 44 in this example is delayed by one delay coil60 from reaching a second sensor 20, and the other delay coil 60 againdelays the light being transmitted back to the detector 46. Of course,any number of delay coils 60 (including one), and any specific delayamount, may be used in keeping with the scope of this disclosure.

Since the light 44 from the different sensors 20 will arrive at thedetector 46 at respective different times, the detector can easilydetermine the light that was transmitted through each set of sensors.Although only two sensors 20 are depicted in FIG. 11, multiple sensorscan be time division multiplexed, depending for example on requiredsystem performance.

In FIG. 12, optical delay coils 60 are used to achieve time divisionmultiplexing, along with the wavelength division multiplexing providedby the WDM filters 56 and couplers 58. The light 44 in this example isdelayed by one delay coil 60 from reaching a second set of WDMmultiplexed sensors 20, and the other delay coil 60 again delays thelight being transmitted back to the detector 46. Again, any number ofdelay coils 60 (including one), and any specific delay amount, may beused in keeping with the scope of this disclosure.

Since the light 44 from the different sets of WDM multiplexed sensors 20will arrive at the detector 46 at respective different times, thedetector can easily determine the light that was transmitted througheach set of sensors. Wavelength differences in the light 44 returned tothe detector 46 characterize particular sensors 20 in each set ofsensors. Although only two sets of WDM and TDM multiplexed sets ofsensors 20 are depicted in FIG. 12, multiple arrangements of sensors maybe used in other examples, based for example on required systemperformance.

It may now be fully appreciated that the above disclosure providessignificant advancements to the art of detecting electromagnetic fieldsin subterranean formations, and thereby measuring resistivities offormations. In examples described above, the sensor 20 includes aninterferometer 36 with both negative and positive magnetostrictivematerials 32, 52, thereby increasing a sensitivity of the sensor tochanges in resistivity of the formation 14, enabling monitoring of theprogress of the flood front 12 from a greater distance, and with greateraccuracy. However, it is not necessary for a flood front to be monitoredin keeping with the scope of this disclosure.

A method of measuring an electromagnetic field 28 in a subterraneanearth formation 14 is provided to the art by the above disclosure. Inone example, the method can comprise: installing at least oneelectromagnetic sensor 20 in a well, the sensor 20 comprising multipleoptical waveguides 30, 50 and respective multiple materials 32, 52; andin response to exposure to the electromagnetic field 28, the materials32, 52 changing shape, thereby increasing optical path length or opticalphase in a first one of the optical waveguides 30 and decreasing opticalpath length or optical phase in a second one of the optical waveguides50.

The changing shape step can include the first optical waveguide 30elongating and the second optical waveguide 50 contracting. Thematerials 32, 52 can comprise both positive and negativemagnetostrictive materials.

The optical waveguides 30, 50 may comprise legs of an interferometer 36.The materials 32, 52 can be bonded directly to the respective opticalwaveguides 30, 50.

The method may include permanently installing the sensor 20 in awellbore 16. The method may include installing the sensor 20 in cement22 between a casing 24 and a wellbore 16.

The method can include the sensor 20 detecting the electromagnetic field28 which is related to resistivity in the formation 14.

Also described above is a well system 10. In one example, the wellsystem 10 can include an optical electromagnetic sensor 20 installed ina well, whereby the sensor 20 measures an electromagnetic field 28 in anearth formation 14. Optical phases or path lengths in optical waveguides30, 50 of the sensor 20 change both positively and negatively inresponse to exposure to the electromagnetic field 28.

The sensor 20 can include materials 32, 52 which change shape inresponse to exposure to the electromagnetic field 28. The optical pathlength/phase in the optical waveguides 30, 50 may change in response tochanges in shapes of the respective materials 32, 52.

The materials 32, 52 can comprise magnetostrictive materials. In anexample described above, the materials 32, 52 comprise both positive andnegative magnetostrictive materials.

A method of monitoring an earth formation 14 is also described above. Inone example, the method comprises: installing at least one opticalelectromagnetic sensor 20 in a wellbore 16 which penetrates theformation 14; and a first optical path length in a first opticalwaveguide 30 of the sensor 20 increasing in response to exposure to aelectromagnetic field 28, and a second optical path length in a secondoptical waveguide 50 of the sensor 20 decreasing in response to exposureto the electromagnetic field 28.

The sensor 20 can include first and second materials 32, 52 which changeshape in response to exposure to the electromagnetic field 28. The firstoptical path length in the first optical waveguide 30 changes inresponse to a change in the first material 32 shape, and the secondoptical path length in the second optical waveguide 50 changes inresponse to a change in the second material 52 shape.

The first material 32 may comprise a positive magnetostrictive material,and the second material 52 may comprise a negative magnetostrictivematerial.

The method can include performing a time-lapse measurement of theelectromagnetic field 28 at the sensor 20 as a function of time. Adifferential signal may be recorded in the time-lapse measurement, thedifferential signal being a difference between an output of the sensor20 prior to a change in the formation 14, and an output of the sensor 20after a change in the formation 14.

Although various examples have been described above, with each examplehaving certain features, it should be understood that it is notnecessary for a particular feature of one example to be used exclusivelywith that example. Instead, any of the features described above and/ordepicted in the drawings can be combined with any of the examples, inaddition to or in substitution for any of the other features of thoseexamples. One example's features are not mutually exclusive to anotherexample's features. Instead, the scope of this disclosure encompassesany combination of any of the features.

Although each example described above includes a certain combination offeatures, it should be understood that it is not necessary for allfeatures of an example to be used. Instead, any of the featuresdescribed above can be used, without any other particular feature orfeatures also being used.

It should be understood that the various embodiments described hereinmay be utilized in various orientations, such as inclined, inverted,horizontal, vertical, etc., and in various configurations, withoutdeparting from the principles of this disclosure. The embodiments aredescribed merely as examples of useful applications of the principles ofthe disclosure, which is not limited to any specific details of theseembodiments.

In the above description of the representative examples, directionalterms (such as “above,” “below,” “upper,” “lower,” etc.) are used forconvenience in referring to the accompanying drawings. However, itshould be clearly understood that the scope of this disclosure is notlimited to any particular directions described herein.

The terms “including,” “includes,” “comprising,” “comprises,” andsimilar terms are used in a non-limiting sense in this specification.For example, if a system, method, apparatus, device, etc., is describedas “including” a certain feature or element, the system, method,apparatus, device, etc., can include that feature or element, and canalso include other features or elements. Similarly, the term “comprises”is considered to mean “comprises, but is not limited to.”

Of course, a person skilled in the art would, upon a carefulconsideration of the above description of representative embodiments ofthe disclosure, readily appreciate that many modifications, additions,substitutions, deletions, and other changes may be made to the specificembodiments, and such changes are contemplated by the principles of thisdisclosure. For example, structures disclosed as being separately formedcan, in other examples, be integrally formed and vice versa.Accordingly, the foregoing detailed description is to be clearlyunderstood as being given by way of illustration and example only, thespirit and scope of the invention being limited solely by the appendedclaims and their equivalents.

The invention claimed is:
 1. A method of monitoring a flood front, themethod comprising: installing at least one optical electromagneticsensor in a wellbore in cement between a casing and the wellbore, thewellbore penetrating an earth formation, the sensor being part of aninterferometer selected from the group consisting of a Mach Zehnderinterferometer and a Michelson interferometer; inducing anelectromagnetic field in the earth formation by energizing the casing,wherein a first optical path length in a first optical waveguide of thesensor increases in response to exposure to the electromagnetic field,and a second optical path length in a second optical waveguide of thesensor decreases in response to exposure to the electromagnetic field;and monitoring the flood front by detecting via the sensor theelectromagnetic field in the earth formation as the flood frontprogresses through the earth formation.
 2. The method of claim 1,wherein the sensor further comprises first and second materials whichchange shape in response to exposure to the electromagnetic field. 3.The method of claim 2, wherein the first optical path length in thefirst optical waveguide changes in response to a change in the firstmaterial shape, and wherein the second optical path length in the secondoptical waveguide changes in response to a change in the second materialshape.
 4. The method of claim 2, wherein the first and second materialscomprise magnetostrictive materials.
 5. The method of claim 2, whereinthe first material comprises a positive magnetostrictive material, andwherein the second material comprises a negative magnetostrictivematerial.
 6. The method of claim 1, wherein the installing furthercomprises permanently installing the sensor in a wellbore.
 7. The methodof claim 1, wherein the electromagnetic field detected via the sensor isassociated with resistivity in the formation.
 8. The method of claim 1,wherein the at least one sensor comprises multiple sensors, and furthercomprising wavelength division multiplexing the sensors.
 9. The methodof claim 1, wherein the at least one sensor comprises multiple sensors,and further comprising time division multiplexing the sensors.
 10. Themethod of claim 1 wherein the at least one sensor comprises multiplesensors, and further comprising wavelength and time divisionmultiplexing the sensors.
 11. The method of claim 10, wherein adifferential signal is recorded in the time-lapse measurement, thedifferential signal being a difference between an output of the sensorprior to a change in the formation, and an output of the sensor afterthe change in the formation.
 12. The method of claim 1, furthercomprising performing a time-lapse measurement of the electromagneticfield at the sensor as a function of time.